N does not contain the name of any variable. N contains the string in your edit box called edit1. If, for some reason, you had your user type the file name of a mat file into that edit box instead of using uigetfile(), then you would do
storedStructure = load(N);
Now, the variable "storedStructure" (which is a structure) will contain all the variables that you saved into the mat file (when you called the save() function) as members ("fields") of that structure variable.
If the string in your edit box is the name of a variable instead of the name of a file, (it's hard to tell exactly from your description), then you'd have to know the name of the mat file somehow and use dynamic structure names to recall it. But this would be an unusual use of save() and load(), as almost always you know in advance what you want to save or recall from the mat file.
